With increasing demand being made upon existing water supplies, conservation of these supplies becomes more important. Not only is it important to effect greater economy in the use of existing water supplies, but it also more and more necessary, as health, conservation and governmental restrictions become more stringent, to process spent or waste liquids prior to dumping them as effluents in sewers, rivers, lakes or other available disposal outlets.
Ammong the methods for the treatment of effluents proposed, electrolysis seems to present one of the best prospects for handling the large volummes of solutions generally encountered but studies of the electrolysis systems employed heretofore have shown that these are efficient in commercial use only for highly concentrates solutions and that they are not satisfactory in reducing pollutant ion concentrations down to the very low order or level required for most purposes.
In a conventional electrolytic cell, direct current is applied to spaced electrodes immersed in the solution undergoing treatment, and the electrical circuit of the system is completed solely through ionization of the solution and migration of the ions to the surfaces of the electrodes. Thus, the current in a conventional electrolytic cell is carried through the solution solely by ion migration. At the surface of the electrode, an electrical charge is transferred between the ions in solution and conductive electrode. At the anode, electrons are lost to the electrode, or oxidation occurs; at the cathode, electrons are gained from the electrode, or reduction occurs. The electrodes thus act as the catalytic surfaces on which the electrochemical reaction takes place, and the reaction is localized, i.e., takes place only at the electrode surfaces. Since the current or flow of electrons within the electrolyte is carried out by the ions, for any given fixed applied potential at the electrodes, the amount of current passing through the system is, in general, proportional to the concentration of the ions present in solution. Hence, as the ion content decreases, the current in the system also decreases, and since the reactions which occur at the electrode are dependent on the flow of electrons, it can be seen that the rate of the reactions decreases with decreasing concentrations. It is also obvious from the above that the resistance or resistivity of the electrolyte itself increases with decreasing concentration of ions present. Thus, for a fixed applied potential, in order to maintain a substantially constant rate of electron flow it would be necessary to decrease the distance between the electrodes as ion concentration decreases. This is generally impractical.
In order to overcome some of the drawbacks of conventional electrolytic cells, Belgian Pat. No. 739,684 and U.S. Pat. No. 3,616,356 disclose the use of bed formed of solid particulate electrically conductive packing elements, which bed has disposed therein at least two spaced electrodes making electrical contact with the packing elements. The composite bed and electrodes are supported in some suitably insluated treatment vessel to which the metal-containing solutions or other impure aqueous fluids to be treated are introduced while direct current of appropriate potential is applied to the electrodes from a suitable power source. Due to contact resistance of the packing elements and at the electrode surfaces, an electrical path of relatively low conductivity is established through the bed and there is produced in this system due to intermittency of contact a phenomenon known as bipolarity in the individual particulate packing elements, thus causing a multiplicity of positive and negative sites to exist within the bed. When a solution or moist gaseous stream containing metal ions is introduced into or passed through the electrically charged bed of the system, an electrochemical reaction occurs at each of such sites whereby positively charged ions are reduced and deposited on or at the negatively charged portion of the bed elements. Additionally, where metal ions are present, precipitation of such ions can and does occur within the vessel due to reaction with other ions whose concentration is influenced by the electrochemical action. Consequently, the metal ion concentration of the solution or gaseous stream can be reduced to a low level.
In practice, however, the processes described in Belgian Pat. No. 739,684 and U.S. Pat. No. 3,616,356 has certain limitations. Thus, the flow rate of the systems disclosed in these references is limited due to several hydraulic factors including (a) the tendency of the electrolyte to channel, i.e., seek paths of low flow resistance through the bed thereby reducing the bed efficiency, and (b) the tendency of the bed to become too tightly packed at greater flow rates thereby plugging the system and eventually causing shorting out through the bed material. As a result of these drawbacks, the efficiency of the system in terms of gallons of effluent treated per hour per cubic foot of bed is severely limited, i.e., normally up to about 15 gal./hr/cu. ft. of bed. This bed efficiency limitation makes it necessary to use larger beds, and consequently more and larger units, in order to treat large volumes of effluent having high contaminant concentrations. This situation is further aggravated by the fact that these systems cannot be improved beyond their hydraulic limitations by linking them in series. As a consequence the only effective method of treating large volumes is by employing a large number of units in parrallel. This procedure is normally too costly for most commercial applications.